Planar arrays of printed elements are widely known, but may not be useful in some applications, such as radar, where high power must be handled. Also, planar arrays of printed elements tend to couple preferentially into the dielectric substrate, and if a dielectric antenna face plate is used, into the faceplate. Under some conditions of off-broadside radiation, total internal reflection can occur, which results in "blind angles" at which radiation does not take place. In addition, an external radome is required to protect a printed-circuit antenna from the external environment. In a harsh environment, some kind of heating mechanism has to be provided in order to prevent formation of ice on a radome surface. This heating requirement tends to make the design of such a radome difficult. For many radar systems, horn antenna arrays are preferred, because of their ruggedness, power-handling capability, and gain. In harsh environments, a dielectric window, preferably ceramic, is placed over the aperture of the horn to prevent ingress of corrosive precipitation and other matter into the horn cavity.
Horn antennas tend to be physically heavier than printed-circuit antennas, and are three-dimensional rather than two-dimensional. Making a large array of horn antenna elements can require a significant construction effort. Among the problems to be overcome are (a) mounting of the horns in close proximity to each other without interference; (b) assuring that the mounted horns have mutually parallel axes; (c) making the requisite connections at the back of the horn array; (d) making sure that the dielectric windows are sealed; and after the antenna is installed, the further problems arise of (e) gaining access to a particular horn of the array for maintenance or replacement; and (f) performing step (e) without allowing the ingress of corrosive precipitation or other matter. In addition to these physical considerations, high-performance antennas require a broad operating bandwidth, preferably a 2:1 frequency bandwidth.
One of the most difficult aspects of the design of a horn array is the requirement for impedance matching in the array environment over the scan volume. Whatever the bandwidth of interest, it is always more difficult to match the antennas in a controllable array to their respective feeds than it is to match an individual antenna alone. The problem arises because, when mounted in the array, each antenna element is subject to mutual coupling from the adjacent antenna elements, which varies in both amplitude and phase in response to beam steering. When the instantaneous bandwidth is large, as for example 2:1, impedance matching is even more difficult.
An improved horn element and array is desired.